Second law of thermodynamics implies the universe is of finite age

Apologists like to argue the universe was created. This is only possible if it has a finite age. Attempts to justify the finite age of the universe have sometimes been based on thermodynamics. The 2nd law of thermodynamics states that entropy or disorder tends to increase with time. In other words, it is easier to break something than to fix it (because there are more ways a system can be broken than a ways a system can be working). It is easier to use oil as fuel than CO2 and water, etc. Increasing entropy corresponds with a decrease in energy that can perform useful work.

"This aspect of the Second Law also tells us that the universe had a beginning. Since we still have some order left-just like we still have some usable energy left-the universe cannot be eternal, because if it were, we would have reached complete disorder (entropy) by now.[1]"

This argument is a little more up to date than insisting infinite regress is impossible.

The source of usable entropy has been used to argue for the existence of God. This is in the standard form of the cosmological argument: [3]

"[Where] did our initial store of useful energy come from? An easy out might be to propose that the universe is not bound to the second law. But this would be contrary to all scientific evidence we have. It seems that the universe needs some supernatural force to provide the initial high energy density that still powers the universe today. [...] The universe needs a cause to exist, it also needs a source for its initial supply of energy. These two facts alone provide compelling evidence for a created universe. But this is only a small part of the whole story. It turns out the universe needs a lot of things from its Creator, to allow it to exist, to have complexity, and to have life."

The small size of the early universe may have limited entropy to be low. This compact universe may have been an effective entropy reset. As it has expanded, by inflation, entropy has been much more "room" to increase. [4]

The second law is a statistical law that predicts that entropy will usually increase. On a macro scale, it basically always does increase. However there is a very slight chance of decrease. Given an infinite time, global entropy will decrease from time to time. This is basically Poincaré recurrence theorem.

There may be an unknown physical process that causes entropy to decrease. For instance the arrow of time could reverse. [5]

"At the very least it would seem wisest, if we no longer dogmatically assert that the principles of thermodynamics necessarily require a universe which was created at a finite time in the past and which is fated for stagnation and death in the future. [6]"

One interesting point is there is no minimum entropy, so entropy could have been increasing for an infinite time until it happened to reach its current state:

"The only person who seems to have taken seriously the simple mathematical truth that the ever-increasing nature of the entropy did not imply that it had to have been zero a finite time ago was the Catholic physicist and historian of science Pierre Duhem (1861-1916). He refused to use it as an argument for the creation of the universe out of nothing in the finite past or the achievement of a total heat death in the future because the continuous increase of the entropy of the universe did not mean it had ever experienced a minimum value, or would ever reach a maximum value in the future. [7]"

The rate of entropy increase could be variable, even to the point of allowing an infinite age of the universe:

"[E]ven if we were to grant that the second law is universally applicable, we could not conclude from it that the universe necessarily began a finite time in the past. All that can be said is that the universal entropy has never been decreasing. For example, it may well be that the entropy has merely increased asymptotically from some definite minimum value in the infinite past and will continue to increase to some maximum value in the infinite future. This would be the case in a cosmological model such as Gamow's infinitely old contraction-expansion universe, described above. Thus the second law of thermodynamics by itself does not provide any independent evidence of a beginning of the universe a finite time ago. It can do that only when used in conjunction with a specific cosmological model. [8]"